Hitherto, as the linear driving ultrasonic motor as a driving apparatus, there have been various inventions made, and for example, PTL 1 or PTL 2 discloses an ultrasonic motor.
PTL 1 describes an example where the amplitudes of two different kinds of bending vibration having the substantially same natural frequency are extracted to the distal ends of projections for driving.
PTL 2 is an improvement of the invention of PTL 1, and describes an example where driving is made by a piezoelectric element with a simple configuration.
A vibrator for use in an ultrasonic motor of the related art will be described referring to FIGS. 8A to 8E. FIGS. 8A to 8E illustrate a configuration of a vibrator for use in an ultrasonic motor of PTL 2. FIG. 8A is a plan view, FIG. 8B is a front view of FIG. 8A, FIG. 8C is a left side view of FIG. 8A, FIG. 8D is a right side view of FIG. 8A, and FIG. 8E is a sectional view taken along the line 8E-8E of FIG. 8B.
The vibrator has a vibration plate 101 having a rectangular shape, and holing parts 101a and 101b which are provided on the shorter sides of the vibration plate 101 for holing to a retention member (not shown). The vibrator further has two projections 103a and 103b which are bonded to the vibration plate 101, and a piezoelectric element 102 which is bonded to a rear surface of a surface to which the projections 103a and 103b of the vibration plate 101 are bonded. The piezoelectric element 102 is polarized into two phases of an A phase 102a and a B phase 102b. 
The projection 103a is bonded to a place of an antinode X3 of a primary natural vibration mode of bending vibration excited by the piezoelectric element 102 and generated in a direction along the shorter side of the vibration plate 101. The projection 103a is also bonded to a place of one node Y3 of a secondary natural vibration mode of bending vibration excited by the piezoelectric element 102 and generated in a direction along the longer side of the vibration plate 101. The projection 103b is bonded to the place of the antinode X3 of the primary natural vibration mode of bending vibration generated in the direction along the shorter side of the piezoelectric element 102. The projection 103b is also bonded to a place of the other node Y4 of the secondary natural vibration mode of bending vibration generated in the direction along the longer side. The two projections 103a and 103b are in contact with a friction member 201 fixed to a fixed frame (not shown) on a surface opposite to the bonding surface to the vibration plate 101. In the above-described configuration, an AC voltage is applied from power feed means (not shown) to the A phase 102a and the B phase 102b of the piezoelectric element 102 while changing a phase difference in a range of −90° to +90°. With this, ultrasonic vibration is generated, and a driving force for relative movement of the vibrator and the friction member 201 is generated.
Subsequently, a mode of the vibrator when an AC voltage having a phase difference is applied to the A phase 102a and the B phase 102b of the piezoelectric element 102 will be described referring to FIGS. 9 and 10.
FIG. 9 models and illustrates a mode of the vibrator when an AC voltage is applied while delaying the phase of the B phase 102b by about +90° with respect to the A phase 102a of the piezoelectric element 102. The piezoelectric element 102 and the holding parts 101a and 101b are omitted. The (a) of FIG. 9 illustrates changes in the AC voltage which is applied to the A phase 102a and the B phase 102b of the piezoelectric element 102, and in the (a) of FIG. 9, the vertical axis represents a voltage and the horizontal axis represents time. A voltage V5 is applied to the A phase, and a voltage V6 is applied to the B phase. The (b) of FIG. 9 is a front view of the vibrator, the (c) of FIG. 9 is a left side view of the vibrator at the bonding position of the left projection 103a of the vibrator, and the (d) of FIG. 9 is a right side view of the vibrator at the bonding position of the right projection 103b of the vibrator in the (b) to (d) of FIG. 9, state change in vibration of the vibrator at the time T9 to the time T12 of the (a) of FIG. 9 is indicated by a solid line. A dotted line indicates the state of the vibrator other than at the time indicated by the solid line for comparison.
FIG. 10 models and illustrates a mode of the vibrator when an AC voltage having no substantial phase difference is applied between the A phase 102a and the B phase 102b of the piezoelectric element 102. The piezoelectric element 102 and the holing parts 101a and 101b are omitted. The (a) of FIG. 10 illustrates changes in the AC voltage which is applied to the A phase 102a and the B phase 102b of the piezoelectric element 102, and in the (a) of FIG. 10, the vertical axis represents a voltage and the horizontal axis represents time. A voltage V7 is applied to the A phase, and a voltage V8 is applied to the B phase. The (b) of FIG. 10 is a front view of the vibrator, the (c) of FIG. 10 is a left side view of the vibrator at the bonding position of the left projection 103a of the vibrator, and the (d) of FIG. 10 is a right side view of the vibrator at the bonding position of the right projection 103b of the vibrator. In the (b) to (d) of FIG. 10, state change in vibration of the vibrator at the time T13 to the time T16 of the (a) of FIG. 10 is indicated by a solid line. A dotted line indicates the state of the vibrator other than at the time represented by the solid line for comparison.
As in FIG. 9, when an AC voltage is applied while delaying the phase of the B phase 102b by about +90° with respect to the A phase 102a of the piezoelectric element 102, at the time T10 and the time T12, as in the (a) of FIG. 9, voltages of the same sign having the same magnitude are applied to the A phase 102a and the B phase 102b. At this time, as in the (c) and (d) of FIG. 9, the A phase 102a and the B phase 102b the most expand and contract in the same direction within the same plane. The amplitude of the primary bending vibration generated in the direction along the shorter side of the vibration plate 101 becomes a maximum (P5). Accordingly, as in the (c) and (d) of FIG. 9, this place becomes the antinode X3 of the primary natural vibration mode of bending vibration generated in the direction along the shorter side of the vibration plate 101.
At the time T9 and the time T11, as in the (a) of FIG. 9, voltages of different signs having the same magnitude are applied to the A phase 102a and the B phase 102b of the piezoelectric element 102. At this time, as in the (b) of FIG. 9, the A phase 102a and the B phase 102b the most expand and contract in opposite directions within the same plane. The amplitude of the secondary bending vibration generated in a direction along the longer side of the vibration plate 101 becomes a maximum (P6). Accordingly, as in the (b) of FIG. 9, this place becomes the antinode of the secondary natural vibration mode of the bending vibration generated in the direction along the longer side of the vibration plate 101. The place where the projection 103a is arranged becomes the node Y3 of the primary natural vibration mode of bending vibration generated in the direction along the shorter side of the vibration plate 101, and the place where the projection 103b is arranged becomes the node Y4 of the primary natural vibration mode of bending vibration generated in the direction along the shorter side of the vibration plate 101.
As a result, since a circular motion R5 is generated at the distal end of the projection 103a and a circular motion R6 is generated a the distal end of the projection 103b, the vibrator obtains a driving force to move in an Xd direction with respect to the friction member 201. When an AC voltage is applied while delaying the phase of the A phase 102a by about +90° with respect to the B phase 102b, since a circular motion in a direction opposite to the circular motion R5 is generated, the vibrator obtains a driving force to move in a direction opposite to the Xd direction with respect to the friction member 201.
As in FIG. 10, when an AC voltage having no substantial phase difference is applied between the A phase and the B phase of the piezoelectric element, at the time T14 and the time T16, as in the (a) of FIG. 10, voltages of the same sign having the same magnitude are applied to the A phase 102a and the B phase 102b of the piezoelectric element 102. At this time, as in the (c) and (d) of FIG. 10, the A phase 102a and the B phase 102b the most expand and contract in the same direction within the same plane. The amplitude of the primary bending vibration generated in the direction along the shorter side of the vibration elate 101 becomes a maximum (P7). Accordingly, as in the (c) and (d) of FIG. 10, this place becomes the antinode X3 of the primary natural vibration mode of bending vibration generated in the direction along the shorter side of the vibration plate 101.
At the time T13 and the time T15, compared to a case where an AC voltage is applied while delaying the phase of the B phase 102b by about +90° with respect to the A phase 102a of the piezoelectric element 102 (FIG. 9), as in the (a) of FIG. 10, the time when voltages of different signs are applied between the A phase 102a and the B phase 102b is very short. Accordingly, as in the (b) of FIG. 10, the secondary bending vibration generated in the direction along the longer side of the vibration plate 101 becomes small compared to a case where an AC voltage is applied while delaying the phase of the B phase 102b by about +90° with respect to the A phase 102a (P8). Therefore, as in the (b) of FIG. 10, this place becomes the antinode of the secondary natural vibration mode of bending vibration generated in the direction along the longer side of the vibration plate 101. The place where the projection 103a is arranged becomes the node Y3 of the primary natural vibration mode of bending vibration in the direction along the shorter side of the vibration plate 101, and the place where the projection 103b is arranged becomes the node Y4 of the primary natural vibration mode of bending vibration generated in the direction along the shorter side of the vibration plate 101.
As a result, since a longitudinal elliptic motion R7 is generated at the distal end of the projection 103a and a longitudinal elliptic motion R8 is generated at the distal end of the projection 103b, the vibrator can obtain a driving force to move in the Xd direction with respect to the friction member 201 at low speed. Similarly to the circular motion R5, a driving force for moving in an opposite direction can be obtained.
In this way, the ultrasonic motor of the related art controls the phase difference of the AC voltage which is applied to the A phase 102a and the B phase 102b of the piezoelectric element 102, thereby changing the elliptical ratio of the motion of the distal end of the projection to correspond to a wide speed range.